This invention relates to wide angle photographic objectives and, more particularly, the invention is concerned with providing improved lens systems suitable for aerial mapping wherein wafer elements of high index flint glass are used next to a central stop and additional wafer elements are introduced for reducing chromatic astigmatism.
The basic form of the wide angle optical objective is described in my U.S. Pat. No. 2,821,113 and improved forms of the lens system are described in my U.S. Pat. No. 3,039,361. The hereinafter presented disclosure presents an improved lens system wherein the chromatic astigmatism and the oblique spherical aberration in the skew direction in the previously patented lens systems are reduced by adopting the more complex structure including the aspherics described below.
In the lens system described in U.S. Pat. No. 2,821,113, the outermost elements are weakly positive menisic, or at least, the first element must be weakly positive. There is thus an initial convergence of the rays before the negative element is introduced, a factor that keeps the physical length of the system within modest proportions. The general concept of strong negative elements lying outside of centrally located strong positive elements is used for the purpose of obtaining favorable illumination over the wide angle field. Ten elements were employed, five on either side of the central stop. Inasmuch as the relative heights through the system on either side are not very different between negative and positive refractions, a circumstance favorable to good oblique performance over a wide field, it proved necessary to introduce one high index type glass on either side to preserve the desired lens power. Also, it should be noted that the negative concentric refraction on either side obtained at a cemented surface, though essential for eliminating a good part of spherical aberration of the system without imparting oblique spherical aberration, does weaken the system. The positive element is necessarily of lower index than the adjacent negative element, so that the dioptric power at the interface is strongly negative. This same circumstance is unfavorable to the Petzval sum, or conversely, for a given low Petzval sum of such a wide angle system, is unfavorable for the net lens power and causes a general run of steep curvatures. The introduction of the high index elements offsets such an adverse effect.
In the lens systems descirbed in U.S. Pat. No. 3,039,361, greater lens speeds are achieved without sacrifice of resolving power, contrast or correction for distortion. In one of these systems a glass pressure plate was introduced at the image plane for the purpose of keeping the emulsion as flat as possible. Introduction of the glass plate greatly upset the symmetry of the design at high angles and to a corresponding sensitivity in the compensating refractions, leading to exaggerated astigmatism at high angles. In another of these systems, an attempt was made to obtain wide angle performance at f/2.5 for a 3-inch focal length on a 4.5 by 4.5-inch format. The goal was reached but at a cost of very enchanced sensitivity to centration errors and to radii. From the foregoing remarks, it can be seen that an improvement of the color correction both longitudinally and with respect to chromatic astigmatism would be desirable in order to provide a mapping lens system more commercially usable by aerial survey companies.